The present invention relates to improvements in galvanic cells, and more particularly to seawater activated batteries.
Galvanic cells are frequently used as seawater activated batteries in sonobuoys to provide electrical power to electrical components after they have been deployed at great depths in the sea. A battery found particularly useful for providing high power pulses required in some applications utilizes a zinc (Zn) anode and a nickel oxide hydroxide (NiOOH) cathode separated by a porous mat with potassium hydroxide (KOH) as the electrolyte for dissolution in the seawater. The KOH is impregnated into the pores of the cathode and the mat to produce the required ion concentration when dissolved in the seawater. Battery shelf life is reduced, however, because the KOH in the mat, wet or dry, reacts with and corrodes the Zn anode. Moreover, inadequate dispersion of the KOH in solution occurs due to blockage or de-activation of active reaction sites on the cathode, increased impedance due to the solid layer of KOH on the cathode surface, localized heating which generates gas bubbles disrupting intimate contact of the electrolyte and anode, and deposition of products of the reaction between the KOH and the seawater.
Maximum conductivity within a galvanic cell occurs within a limited range of electrolyte concentration. With a Zn anode and NiOOH cathode, this conductivity occurs at a concentration of approximately 27 to 30 percent by weight KOH in seawater. When the KOH is impregnated into the cathode and the separating mat, a relatively large concentration gradient also results between the cathode and anode. Near the cathode, the concentration is considerably above 30%; and near the anode, it is considerably below 27%. Consequently, poor and variable battery performance results.